Recently, a display device driven by a driving unit using an indium gallium zinc oxide a-IGZO which is an oxide semiconductor has been rapidly developed. Additionally, study about an inverter needed basically for driving the display device and a driving circuit using the inverter has been considerably progressed.
Korean laid open patent No. 10-2012-0087910 as a related technique discloses an oxide semiconductor thin film transistor having a etch/stopper E/S typed dual gate structure.
However, in the conventional oxide semiconductor thin film transistor, a bottom gate and a top gate are electrically separated, and a voltage is not applied to the top gate.
On the other hand, the oxide semiconductor thin film transistor may operate with a depletion mode in the event that specific voltage is applied to the top gate. However, the problem exists in that electrical characteristics of the transistor is deteriorated if a voltage applied to the top gate is different from that applied to the bottom gate.
A parasitic voltage occurs between the top gate and a source electrode/a drain electrode, thereby deteriorating characteristics of the oxide semiconductor thin film transistor having high performance electrical characteristics.
Furthermore, the problem exists in that electrical characteristics of the a-IGZO thin film transistor are terribly changed in view of negative bias illumination stress NBIS which is one of reliability testing of the oxide semiconductor thin film transistor.